我5438外接25MHz, 跑起来未发现问题。
void initClock(void)
{
  
  // 初始化P7.0(#13)和P7.1(#14)为复用功能, XT1 外部时钟晶体接线
  GPIO_setAsPeripheralModuleFunctionInputPin( GPIO_PORT_P7 , GPIO_PIN0 | GPIO_PIN1 );  
  // 启动XT1  
  //Initializes the XT1 crystal oscillator with no timeout
  //In case of failure, code hangs here.
  //For time-out instead of code hang use UCS_LFXT1StartWithTimeout()
  UCS_LFXT1Start( UCS_XT1_DRIVE0 , UCS_XCAP_3);   
  // DCO参考时钟选择XT1，选择了默认参数  
  //UCS_clockSignalInit(  UCS_FLLREF, UCS_XT1CLK_SELECT , UCS_CLOCK_DIVIDER_1 );
  
  
  // 初始化P5.2(#89)和P5.3(#90)为复用功能, XT2 外部时钟晶体接线
  GPIO_setAsPeripheralModuleFunctionInputPin( GPIO_PORT_P5 , GPIO_PIN2 | GPIO_PIN3 );  
  // 启动XT2
  UCS_XT2Start( UCS_XT2DRIVE_24MHZ_32MHZ );  
  
  // DCO参考时钟选择XT1，选择了默认参数  
  //UCS_clockSignalInit(  UCS_FLLREF, UCS_XT1CLK_SELECT , UCS_CLOCK_DIVIDER_1 );
  
  
  // ACLK参考时钟源选择REFO, 32768Hz
  //UCS_clockSignalInit(UCS_ACLK, UCS_REFOCLK_SELECT,UCS_CLOCK_DIVIDER_1);   
  // DCO参考时钟源选择REFO，32768Hz
  //UCS_clockSignalInit(UCS_FLLREF, UCS_REFOCLK_SELECT , UCS_CLOCK_DIVIDER_1);
  
  // DCO时钟16MHz, used as MCLK and SMCLK frequency
  // 该函数第1个参数为系统工作频率 16384KHz/1024=16MHz，第2个参数为 工作频率/DCO参考频率=16*1024*1024/32768=512
#define FRQ_DIV 1
  //UCS_initFLLSettle(16384/FRQ_DIV , 512/FRQ_DIV);  
  
  //! Initializes the XT2 crystal oscillator, which supports crystal frequencies
  //! between 4 MHz and 32 MHz, depending on the selected drive strength. Loops
  //! until all oscillator fault flags are cleared, with no timeout. See the
  //! device-specific data sheet for appropriate drive settings.
  UCS_clockSignalInit(UCS_MCLK,UCS_XT2CLK_SELECT ,UCS_CLOCK_DIVIDER_1);
  
  
  //SMCLK参考时钟源选择DCO,16MHz/4=4MHz,4*1024*1024=4194304Hz
  //UCS_clockSignalInit(UCS_SMCLK,UCS_DCOCLK_SELECT,UCS_CLOCK_DIVIDER_4);
  UCS_clockSignalInit(UCS_SMCLK,UCS_XT2CLK_SELECT,UCS_CLOCK_DIVIDER_4);
  
  //延时0.5秒, 让时钟源稳定
  delay_ms(500);
  
   UCS_setExternalClockSource(32768,MCLK_FRQ);  
  
  cvMCLK = UCS_getMCLK();  
  cvSMCLK = UCS_getSMCLK();  
  cvACLK = UCS_getACLK();  
  
}